1. Technical Field of the Invention
This invention relates to the construction of piles and, in particular, relates to a tool for enlarging a portion of an underground pile shaft.
2. Prior Art
The process of forming an enlarged portion in a pile shaft, known as a ream or an underream, is well known. The benefits of the enlarged surface can be seen in the contribution to the end bearing capacity of the pile and, furthermore, the friction experienced between the surface of the resultant pile and the surrounding soil is enhanced, thereby contributing to the overall load bearing capacity of the pile.
The process of underreaming piles is long established. Where the ground is stable, for example in stiff clays, a pile shaft is usually bored to a given designed depth and then a tool is inserted which is used to enlarge the pile shaft. This tool is conventionally cylindrical in form, and known designs comprise two long open slots which are located in diametrically opposed positions of the cylindrical body side. Within each slot is a cutting arm, which is most commonly hinged at the top. Prior designs are also known where the cutting arms are hinged at the bottom. In both cases, the cutting arms are connected by a linkage to the drilling or Kelly bar of a rotary boring machine so that when the bar is moved downwards and the base of the reaming tool rests on the bottom of the hole, the arms are forced outwardly. On retraction of the drilling bar, the arms are also retracted. While the arms are extended from the underreamer body and are pressing against the surrounding soil, the tool is rotated by the drilling bar so that a soil cutting action is performed. Soil is drawn into the body or centre part of the tool and from time to time the tool is withdrawn from the hole in the ground where the cut soil is removed.
Underreaming is carried out, for example, in forming ground anchors in clay where the objective is to apply loads to the anchor which tend to withdraw it from the ground, so that the process of cleaning the bottom of the shaft is unimportant and does not affect the performance of the anchor. Typically, holes of up to 300 mm diameter are used for ground anchors. In contrast, where a pile is to be constructed, the downward load carried by the pile is transmitted to the ground through the base of the pile shaft, and consequently it has been found that base cleaning of such pile shafts is critical in assuring satisfactory and consistent performance in piles.
In United Kingdom patent GB 2222621, an underreamer is described which comprises a cylindrical body having at least on slot through which a cutting member can protrude. The tool was designed to enable the cutting member to be retracted inside the cylindrical body while the tool is lowered into, or raised from, a pre-formed shaft. An elongate container is also provided below the cutting member so as to collect the spoil generated during the cutting of the surrounding shaft.
However, the volume of spoil generated will invariably be greater than the volume of the material before it is cut from the pile shaft. The so-called “bulking” of the material is dependent on a number of factors including soil type, cutting technique and tool configuration.
It has been found however, that currently available tools for enlarging a portion of an underground shaft, suffer from the disadvantage that in many instances, the spoil generated during the shaft enlarging process, either falls to the bottom of the shaft, or does not all fit in the container if one is provided. For example, when using the tool described in GB 2222621, it has often been necessary to carry out a number of separate operations in order to form the required enlarged portion. In order to alleviate this problem, the tool is initially employed with the cutting arms only partially extended so that a fraction of the required ream is cut. The container is then raised and emptied and the process repeated with the cutting arms extended to a further extent.
Alternatively, larger containers have been employed to accommodate the volume of spoil generated. This can however be a disadvantage since the cutting part of the tool cannot be lowered close enough to the base of the pile shaft to be of maximum benefit.
Reliability of installation requires that excess spoil is extracted from the shaft. Spoil which remains in the shaft until the tool is extracted, and which is not accommodated in a container, will fall to the bottom of the bore. Consequently the tool cannot be inserted to the same depth and therefore an enlarged surface cannot be formed at the base of the bore. This can also cause a ream or underream to perform poorly because the material which rests on the bore surface does not have the same strength and stiffness as the uncut, compacted material. This can significantly affect the bearing capacity of the resulting foundation element.